Field of the Invention
The present invention relates to a photoacoustic device.
Description of the Related Art
The research of optical imaging devices in which a living body is irradiated with light from a light source such as a laser, and information on the inside of the living body which is obtained on the basis of the incident light is represented as an image has been actively advanced. Photoacoustic tomography (PAT) is one of such optical imaging techniques. In the photoacoustic tomography, a living body is irradiated with pulsed light generated from a light source, and a photoacoustic wave (typically, ultrasonic wave) generated from a living body tissue which has absorbed the energy of the pulsed light propagated and diffused inside the living body is detected. The obtained signals are subjected to mathematical analysis processing (image reconstruction processing) with an information processing device, and information relating to optical property values of the object interior is visualized. As a result, it is possible to obtain information on the object interior, for example, the initial sound pressure, optical property values (in particular, the light energy absorption density and absorption coefficient) and distributions thereof, and this information can be used for specifying the distribution of absorbents in the living body and locations of malignant tumors.
In PAT, the initial sound pressure P0 of the acoustic wave generated from a light absorbent inside an object can be represented by the following mathematical formula (I).P0=Γ·μa·Φ  (I)
In this formula, Γ is a Gruneisen parameter, which is obtained by dividing the product of the volume expansion coefficient β and the square of the speed of sound c by the heat capacity Cp at constant pressure. Γ is known to have a substantially constant value for the same object. Further, μa is the absorption coefficient of a light absorber, and Φ is the quantity of light at the position of the light absorber (quantity of light by which the light absorber is irradiated; also referred to as “light fluence”). The initial sound pressure P0 generated by the light absorber inside the object propagates as an acoustic wave inside the object and is detected by an acoustic detector arranged on the object surface. Changes with time of the detected sound pressure of the acoustic wave are detected, and the initial sound pressure distribution P0 can be calculated from the measurement results by using an image reconstruction technique such as back projection. By dividing the calculated initial sound pressure distribution P0 by the Gruneisen parameter Γ, it is possible to obtain the distribution of the product of μa and Φ, that is, the light energy density distribution. Further, where the light quantity distribution Φ inside the object is known, the absorption coefficient distribution μa can be obtained by dividing the light energy density distribution by the light quantity distribution Φ. Thus, where an absorber having known optical properties with respect to the light by which the object is irradiated is introduced as a contrast agent, an ultrasound signal corresponding to the amount of the contrast agent present can be acquired.
Assuming that a living body is the object, blood (hemoglobin) is an absorber with a high degree of absorption of near-IR radiation. Thus, where measurements on a living body are performed by PAT using near-IR radiation, information relating to spatial distribution of blood can be obtained. However, when a sufficient signal contrast (S/N) cannot be obtained only with the amount of hemoglobin present, where a light absorber is further introduced as a contrast agent, it is possible to obtain a PAT image with the S/N enhanced according to the concentration at which the light absorber is present. Contrast agents for blood vessels, which can be used in a living body, are required to be safe with respect to the living body and to have optical properties enabling effective absorption of light in a wavelength region with a high transmissivity in the living body.
For example, Indocyanine Green (abbreviated hereinbelow as ICG) is a contrast agent for blood vessel imaging that demonstrates the above-described performance. Since ICG is a light absorber that produces little adverse effect when a human body is irradiated and that effectively absorbs light in a near-IR wavelength region where high transmissivity in a living body is demonstrated (Gilbert R. Cherrick, Samuel W. Stein, Carroll M. Leevy, Charles S. Davidson, “INDOCYANINE GREEN: OBSERVATIONS ON ITS PHYSICAL PROPERTIES, PLASMA DECAY, AND HEPATIC EXTRACTION”, J Clin Invest., 1960 April; 39 (4): p. 592-600; referred to hereinbelow as Cherrick et al.), it can be advantageously used as a contrast agent in a PAT device.
In this respect, the problem which remains to be solved is that it is difficult to distinguish between an image signal component based on a photoacoustic wave from hemoglobin and an image signal component based on a photoacoustic wave from a contrast agent, and only information in which the two image signal components are mixed can be acquired.
With the foregoing in view, it is an objective of the present invention to provide a photoacoustic device that can acquire more accurate object information.